In various areas of downhole drilling, for example core drilling, a spearhead assembly is attached to a downhole tool to facilitate connection of that tool to an overshot which in turn is attached to a wireline. This allows the tool to be lowered into a drill hole and subsequently retrieved.
The spearhead assembly has a base in the shape of a squat cylindrical having a diametrically extending slot at one end in which one end of spearpoint is pivotally coupled. An opposite end of the spearpoint is configured for releasable latching to the overshot. When connecting a spearhead assembly to an overshot above ground, an operator can easily align the spearpoint and the overshot to ensure coupling. The tool can then be lowered through the drill string by virtue of the coupling of the overshot to the spearhead assembly. When the tool has reached the desired location, the overshot can be decoupled from the spearhead assembly and withdrawn from the drill string to allow drilling to proceed. When it is necessary to withdraw the tool from the drill string, the overshot is again lowered into the drill string for coupling with the spearhead assembly.
The downhole coupling of the overshot and the spearhead assembly is generally reliable. However due to its pivot coupling, from time-to-time the spearpoint can become locked against an internal surface of the drill string and thus cannot be engaged by the overshot to allow withdrawal of the tool. In this instance, it is necessary to withdraw the entire drill string in order to retrieve the tool.